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what are your goals? Just to have fish? To grow the MOST amount of fish? To readily be able to breed said fish? (let them breed themselves or actively help them)
catfish are a great choice for a small set up, but they can be (not will be) tricky to breed in captivity. Depends on many variables. catfish are pretty good at food conversion, and can handle a bit higher stocking densities then most.
Perch should breed well enough on their own but they will need a solid supply of carnivorous sources of food.(insects and smaller fish) the link i posted, he said hes got perch that were trained to take to supplemental feeding. they arent great at feed to meat conversion
bluegill and sunfish that chickencanoe mentioned convert food decently well, but take two years to grow to eating size. they will breed for themselves, so thats easy as well. overall a pretty good choice. depending on what you want. Pretty decent food to meat conversion.
I still like common carp. Not grass carp. grass carp you have to go to lengths to breed in a small pond, they need moving water. youd need to use hormones to breed them. also they do grow fast but eat a TON, not nearly as good at feed to meat conversion as youd think. Common carp CAN be tasty, if they have good water, and the right diet. You can also catch them and keep them in pure water alive a few days, if they are from poor water to purge them of the taste most types of bottom feeders will get in poor water..... If you get israeli or mirror carp they will out grow any fish listed here by leaps and bounds. they will breed on their own, can be supplementaly fed or not. the only fish that competes with food to meat conversion is tilapia that Im aware of, and those need warm water, and as I outlined a few posts back a couple other things the carp trump them on.
so it really depends what you want out of it.... How much space you have etc.... whether you want a more involved or passive set up.... theres a huge stigma on common carp, but it can be a VERY tasty fish, depending on a few variables. the stigma comes from the environments and foods they eat in the wild. Its been a cultured fish for a few thousand years... romans spread the practice across europe out of asia. and monks in europe are actually the ones who bred the mirror carp (a selection of common carp) Israeli carp are bred out of those by aqua culturalists in israel. for even faster growth, and several other things.